Or to decock it with another method, surely?
No. The whole point is to fire the gun safely, if there happens to be a round in the chamber after you cleared it (and failed somehow like the person in the OP's post). Just decocking it will not make the weapon safe.
The vast majority of pistols with decocking levers are double-action pistols. The function of the decocking lever is not part of the clearing process, but to safely decock a loaded weapon so it can be carried in a loaded state more safely. In other words you load the weapon, decock it and put it in your holster, ready to shoot (since it is double-action). Single action pistols like the Glock or Colt 1911 typically have no decocking feature because it would be pointless.
I feel I must repeat for added clarity: The whole point of pulling the trigger (as the last step of clearing a weapon) is to fire the weapon (if it happens to be loaded still).
Slide locked open without a magazine I meant, same as your drill.
A removed magazine does not mean the chamber is empty. I could remove the mag, pull back the slide and lock it, and there could still be a round in the chamber. Sometimes the extractor fails. Such a weapon if dropped would, in fact, most likely result in the slide lock failing and releasing the slide, closing the action and creating a very dangerous situation. Less so in the Glock than most other handguns though because of its internal safeties, but still very undesirable.
Also, your method only works if I witness you dry firing it. If you do that and put the weapon on the bench, then go for a wizz and I come along, I have to assume the weapon is in a dangerous condition (according to the four rules), since I didn't see you dry fire it. Right?
You should always clear the weapon and demonstrate that it is cleared before handing it over to someone else. That person should then immediately clear it as well just to be safe. Any weapon you pick up should be immediately cleared by you no matter what.